The present invention relates to a process for the separation of hydrogen cyanide from gases and waste gases by a through-flow or circulation scrubbing with an aqueous scrubbing liquid to which formaldehyde is added at a controlled rate for conversion of the hydrogen cyanide into glycol nitrile. This process, which is intended in particular for the treatment of gases from coking plants, pressure gasifications of carbon, refuse pyrolysing plants, blast furnace processes and other reductive combustion processes, not only completely separates off the hydrogen cyanide but at the same time prevents overdosing with formaldehyde.
Gases such as, for example, the gases from coking plants and other gases obtained from the pressure gasification of carbon and from blast furnace processes and refuse pyrolysis must be freed from dust before they are used as fuel gases. This is necessary firstly to conform to the regulations governing large scale firing plants concerning dust emissions and secondly to prevent incrustations in the heating furnace as these would severely reduce the efficiency.
To remove the dust from such gases, it is common practice to first pass the gases through cyclones and then through electric filters and finally subjecting the gases to a wet scrubbing. The pH which is established in the scrubbing water may vary from slightly acid to alkaline, depending on the contents of the gases to be purified.
If the gases to be purified contain hydrogen cyanide, hardly any of this impurity is removed at an acid pH and most of it remains in the fuel gas. If, on the other hand, a slightly alkaline pH becomes established in the gas scrubbing medium, part of the hydrogen cyanide will be absorbed but the major proportion of the HCN remains in the gas. When gases which have been purified in this way are used for combustion, the HCN preferentially burns to NO.sub.x, whereby the permissible limits for large scale firing plants; are exceeded and it would only be possible to keep within the allowed limits by adding a DENOX treatment plant.
One possible method of completely removing HCN consists of scrubbing carried out at a strongly alkaline pH. Since, however, all the CO.sub.2 in the gas would have to be scrubbed out at the same time, this method would require an immense consumption of chemicals, which would not only entail very high costs but would also increase the salt content of the effluent to an unacceptable degree.
It has been found that HCN can be separated from gases both in an acidic and in an alkaline pH range if formaldehyde is added to the scrubbing circulation. DE-PS 35 34 677 describes a process for the treatment of effluent from gas scrubbers in which the cyanide specific redox potential is measured in the presence of Ag ions in a side stream which has been conditioned to the required pH, and formaldehyde is introduced into the main stream in an amount depending on the deviation measured so as to convert the HCN dissolved in the scrubbing water and the free cyanide into glycol nitrile. If, however, the scrubbing water also contains sulphide ions, as is the case when scrubbing is carried out on H.sub.2 S-- containing gases from combustion processes operated under reducing conditions, and if it contains thiocyanate, the presence of only 1 mg of sulphide per liter in combination with thiocyanate (which can easily form from cyanide and sulphide) is sufficient to produce a redox potential corresponding to a concentration of 1 g of CN/liter. In that case, overdosing of formaldehyde would occur and this would entail additional costs and result in an unacceptable increase in the CSB. It must also be remembered that free formaldehyde is toxic to fishes and bacteria.
The above-mentioned document also discloses an oxidative treatment of effluent containing glycol nitrile with hydrogen peroxide, which may be carried out either in the scrubbing circulation or in the effluent flushed out of the circulation.
It is an object of the present invention to improve the process disclosed in DE-PS 35 34 677 so that not only hydrogen cyanide will be removed quantitatively from the gases to be scrubbed but overdosing with formaldehyde will be reliably prevented. The process should be suitable in particular for the treatment of gases which contain hydrogen sulphide in addition to hydrogen cyanide. It was also desired to devise a further principle for controlling the dosing of formaldehyde.